Achieving wide band frequency coverage in conventional voltage controlled oscillators (VCOs) requires that the VCO have a wide tuning voltage range. This is often difficult to implement and also creates modulation-related problems in VCOs that are intended to be used for frequency modulation (either digital or analog). The implementation difficulties arise from voltage supply limitations and the modulation related problems arise due to the fact that a single tank circuit is used to determine the center frequency of the oscillator. In order to achieve a wide frequency range while maintaining a reasonable level of modulation sensitivity, i.e., tuning factor, a consequently wide tuning voltage range is required. VCOs having very wide tuning ranges are, however, generally difficult to construct. Usually, in order to achieve a wide frequency tuning range with a reasonable control voltage range, a VCO having a high tuning/modulation factor (in units of MHz/V) must be used.
A consequence of using a high tuning factor, i.e., a limited tuning voltage range, is that the tuning input of the VCO becomes highly sensitive to additive noise and more prone to extraneous signal pick up. This causes the phase noise level of the oscillator to increase resulting in the degradation of the performance of the system that incorporates the VCO. In addition, costly shielding may be required to prevent pick up of such noise.
When a modulating baseband signal is to be summed directly with the tuning voltage signal, or indirectly through a separate port affecting the center frequency of the oscillator's tank circuit, the resultant frequency deviation may vary as a function of the oscillator center frequency. The oscillator's center frequency is determined by the average, i.e., DC value, of the control voltage applied to the modulation/tuning input of the VCO. Note that both analog frequency modulation and digital Frequency Shift Keying (FSK) can be achieved by summing the tuning voltage signal and the baseband signal representing the input data signal.
In the majority of applications it is undesirable to have different tuning sensitivities at different points in the tuning range of the oscillator. Thus, the use of simple wide band oscillators is precluded as such devices typically exhibit this undesirable characteristic. Avoiding an oscillator having different tuning sensitivities within the tuning range typically requires the use of complicated circuitry or the addition of linearization circuits to alleviate the linearity problems associated with tuning and modulating a conventional oscillator over a wide frequency range.
One solution to this problem is to construct a VCO to have a narrow range of frequency coverage with a limited tuning voltage range, but which is capable of switching between frequency bands. The ability to switch between frequency bands permits the VCO to maintain a reasonable tuning/modulation factor within each of the frequency bands. This is typically realized by switching various components such as capacitors, inductors or variable-capacitors that are used in the tank circuit of the oscillator in and out of the circuit. The switching of the components causes the VCO to shift from one band to another. Thus, the frequency tuning range of the VCO is extended without extending the actual tuning voltage range (on a single variable capacitance diode for example) and without imposing a high tuning factor. Note that in addition to the switching elements themselves, e.g., RF PIN diodes, additional capacitors, inductors or variable capacitance diodes are typically necessary.
Oscillators with the capability of switching resonant frequencies are known in the art. U.S. Pat. No. 4,694,262, issued to Inoue et al., discloses an oscillation circuit consisting of a resonator and having a frequency switching means for switching the oscillation frequency of the resonator. The switching element used is a diode.
U.S. Pat. No. 4,536,724, issued to Hasegawa et al., discloses a VCO having an LC resonant circuit which includes a varactor circuit configured so as to control the resonant frequency by means of a DC bias control voltage applied to the varactor circuit.
Various types of well known oscillator circuits can be modified to achieve oscillation and frequency range shifts. Presented below are examples of prior art oscillators adapted to provide frequency range shifting. More detailed descriptions on the various prior art oscillators discussed below can be found in Chapter 5 of H. L. Krauss, C. W. Bostian and F. H. Raab, Solid State Radio Engineering, John Wiley, 1980.
A schematic diagram illustrating a prior art grounded base Colpitts oscillator adapted to provide frequency range switching is shown in FIG. 1. The basic oscillator circuit, generally referenced 10, comprises a transistor 32 and resonant circuit that consists of capacitors 24, 26, 30 and inductor 28. Applying a tuning voltage via resistor 14 to varactor 18 in series with capacitor 16 varies the frequency of oscillation. The frequency of oscillation can be modulated via a modulation input signal by varying the capacitance coupled to the emitter. A modulation input is applied to a varactor 36 in series with capacitor 34 and the emitter of transistor 32. The RF output of the circuit is the emitter voltage.
Note that the circuit described above is one representative possibility. In practice, the modulator circuit comprising capacitor 34, varactor 36 and resistor 38 can be connected across any frequency determining impedance, e.g., inductor 28 and capacitors 30, 34, 24 or 26.
The operating point of the transistor 32 is defined by a voltage divider (not shown) that defines the bias voltage of the transistor base. With this bias system and supply of collector voltage, the transistor is placed in an operating state in which it can provide amplification. Feedback capacitors connected from collector to emitter and from emitter to ground create a state in which connection of a parallel tuned circuit from collector to ground will give rise to electrical oscillations and the circuit becomes an oscillator. Since the feedback capacitors are effectively in parallel with the tuned circuit, the resultant capacitive loading greatly restricts the available tuning range available from this type of oscillator.
The shift in the frequency range is accomplished by switching a range shift capacitor 20 in and out of the circuit. The switching action is performed by diode 22, which may be a PIN, or regular diode. A range control signal is applied via resistor 12. When the range control signal is high, the diode is forward biased and functions to couple the capacitor 20 to ground. Adding the capacitance 20 to the circuit causes a shift in the resonant frequency. To remove the capacitor, a high impedance is applied to the range control input. During the positive half cycle of the RF, the capacitor 20 charges providing a current through forward biased diode 22. During the negative half cycle of the RF, the capacitor cannot discharge as the diode is reverse biased. After a few cycles of RF, the capacitor is charged and the current is reduced. Eventually, the current stops and the diode is no longer forward biased. At this point, the diode and the capacitor are effectively out of the circuit.
This may also be accomplished by applying a negative voltage at the range control input. It is important to note that the value of capacitor 20 is critical to the oscillation frequency. Capacitor 20, together with the other resonant components, determines the frequency of oscillation. Thus, to achieve accurate frequencies, a precision capacitor must be used which increases the cost of the circuit.
Note also that the schematic diagrams shown in FIGS. 1 through 5 present the AC equivalent circuit. The DC biasing has been left out for clarity sake.
A schematic diagram illustrating a prior art grounded base Hartley oscillator adapted to provide frequency range switching is shown in FIG. 2. The oscillator circuit, generally referenced 40, comprises a transistor 58 and a resonant circuit consisting of tapped inductor 54 and capacitors 56, 60. A varactor 48 in series with capacitor 46 provides tuning of the oscillation frequency. A tuning voltage is applied to the varactor 48 via resistor 44. The frequency of oscillation can be modulated via capacitor 60 coupled to the emitter of transistor 58 and in series with varactor 62. The modulation input signal is applied to the cathode of varactor 62 via resistor 64. The voltage on the emitter serves as the RF output.
The frequency range shift is accomplished by switching a capacitor 50 in and out of the circuit. The switching action is performed by diode 52, which may be a PIN, or regular diode. A range control signal is applied via resistor 42. When the range control signal is high, the diode is forward biased and functions to couple the capacitor 50 to ground. Adding the capacitance 50 to the circuit causes a shift in the resonant frequency. A negative voltage or high impedance is applied to the range control input to effectively remove the capacitor from the circuit.
Similarly with the circuit of FIG. 1, the value of capacitor 50 is critical to the oscillation frequency. Capacitor 50, together with the other resonant components, determines the frequency of oscillations thus requiring a precision capacitor to be used.
A schematic diagram illustrating a prior art grounded base Clapp oscillator adapted to provide frequency range switching is shown in FIG. 3. The basic oscillator circuit, generally referenced 70, comprises a transistor 90 and resonant circuit which consists of capacitors 84, 86, 88 and inductors 76, 92. Applying a tuning voltage via resistor 74 to varactor 78 that is in series with inductor 76 varies the frequency of oscillation. The frequency of oscillation can be modulated via a modulation input signal by varying the capacitance coupled to the emitter. A modulation input signal is applied to a varactor 98, which is in series with capacitor 94 and the emitter of transistor 90. The RF output of the circuit is taken from the collector.
Inductor 89 together with capacitor 88 form a parallel resonant circuit functioning as a harmonic band pass filter. This enables the oscillator to output a signal at a desired harmonic of the frequency at which the oscillator oscillates. This is useful especially when a desired frequency at the output cannot be easily realized for practical reasons when using this type of resonator.
The shift in the frequency range is accomplished by switching a range shift capacitor 80 in and out of the circuit. The switching action is performed by diode 82, which may be a PIN, or regular diode. A range control signal is applied via resistor 72. When the range control signal is high, the diode is forward biased and functions to couple the capacitor 80 to ground. Adding the capacitance 80 to the circuit causes a shift in the resonant frequency. A high impedance or negative voltage is applied to the range control input to effectively remove the capacitor from the circuit.
It is important to note that the value of capacitor 80 is critical to the oscillation frequency. Capacitor 80, together with the other resonant components, determines the frequency of oscillations. Thus, to achieve accurate center frequencies, a precision capacitor must be used which increases the cost of the circuit.
A schematic diagram illustrating a prior art modified Clapp oscillator adapted to provide frequency range switching is shown in FIG. 4. The basic oscillator circuit, generally referenced 100, comprises a transistor 120 and resonant circuit, which consists of capacitors 116, 118 and inductor 114. Applying a tuning voltage via resistor 104 to varactor 108, which is in series with capacitor 106, varies the frequency of oscillation. The frequency of oscillation can be modulated via a modulation input by varying the capacitance coupled to the emitter. A modulation input is applied to a varactor 126, which is in series with capacitor 124 and the emitter of transistor 120. The RF output of the circuit is the emitter voltage.
The oscillator 100 is also based on a transistor amplifier. In this circuit, however, the tuning of the circuit is achieved by subtracting the capacitive reactance of the voltage variable capacitance of the tuning diode 108 from the inductive reactance of the tuning inductor 114. The net result being an inductive reactance that resonates with the feedback capacitors as a parallel tuned circuit.
The shift in the frequency range is accomplished by switching a range shift capacitor 110 in and out of the circuit. The switching action is performed by diode 112, which may be a PIN, or regular diode. A range control signal is applied via resistor 102. When the range control signal is high, the diode is forward biased and functions to couple the capacitor 110 to ground. Adding the capacitance 110 to the circuit causes a shift in the resonant frequency. A negative voltage or high impedance is applied to the range control input to effectively remove the capacitor from the circuit.
It is important to note that the value of capacitor 110 is critical to the oscillation frequency. Capacitor 110, together with the other resonant components, determines the frequency of oscillations. Thus, to achieve accurate center frequencies, precision capacitors must be used, which increases the cost of the circuit.
It is important to note that the quality factor and the precision of the capacitor values shown in the circuits of FIGS. 1 to 4 are critical to the operation of the oscillators and have cost implications.